The present invention relates to a frequency converter using a field effect transistor (FET).
A transceiver for use in mobile telecommunications sets or TV receivers needs a frequency converter. The frequency converter converts a radio frequency (RF) signal with a high frequency ranging from 1 to 20 GHz into an intermediate-frequency (IF) signal with a frequency ranging from 10 to 1,000 MHz, or vice versa, using a local oscillator (LO) signal.
To cope with recent upsurge in number of cellular phone users, demand for digital signal transmission and reception techniques has been steeply rising in the field of mobile telecommunications. Also, digital broadcasting has been rapidly popularized to meet the strong demand for multi-channel satellite or ground wave telecasting. For these purposes, the distortion involved with a frequency converter should be reduced as much as possible using FET""S.
Frequency converters with an FET are classified into several types depending on the combination of LO, RF and IF signals with the three input terminals of the FET, namely, source, drain and gate terminals.
Among these various types, a type of frequency converter, in which the LO and RF signals are input to the gate and drain terminals of an FET and the IF signal is output through its drain terminal, is most preferable, because such a converter attains presently lowest possible distortion.
Hereinafter, a prior art frequency converter of this type will be described with reference to FIG. 6.
FIG. 6 illustrates a circuit configuration for the prior art frequency converter. As shown in FIG. 6, the gate terminal 1a of an FET 1 is connected to a first terminal 3, to which an LO signal is input, via an LO matching circuit 2. The drain terminal 1b of the FET 1 is connected to not only a second terminal 5 through an RF matching circuit 4 but also a third terminal 7 by way of an IF matching circuit 6. And the source terminal 1c of the FET 1 is grounded. The impedances of the LO, RF and IF matching circuits 2, 4 and 7 have been optimized in accordance with the frequencies of their associated LO, RF and IF signals, respectively.
Suppose this frequency converter is applied to downconversion, version, i.e., to convert a signal with a relatively high frequency into a signal with a relatively low frequency. In that case, the RF signal, which has been input to the second terminal 5, is converted into the IF signal using the LO signal that has been input through the first terminal 3, and then output through the third terminal 7. Conversely, suppose this frequency converter is applied to upconversion, i.e., to convert a signal with a relatively low frequency into a signal with a relatively high frequency. In that case, the IF signal, which has been input to the third terminal 7, is converted into the RF signal using the LO signal that has been S input through the first terminal 3, and then output through the second terminal 5.
Next, it will be described how the conventional frequency converter operates as a downconverter.
First, an LO signal with an alternating voltage, which has been input through the first terminal 3, is passed through the LO matching circuit 2 and then input to the gate terminal 1a of the FET 1. The FET 1 serves as a switch, which turns ON when the LO signal is positive and turns OFF when the LO signal is negative. Also, there is a channel resistor Rd▪ (not shown) inside the FET 1. The channel resistor Rd▪ functions as a nonlinear resistor having a resistance changing nonlinearly with time. Accordingly, when a relatively high alternating voltage (i.e., the LO signal) is applied to the gate terminal 1a of the FET 1, the RF signal, which has been input to the drain terminal 1b of the FET 1, is converted into the IF signal due to the existence of the nonlinear channel resistor Rd▪. Then, the IF signal is output through the third terminal 7. Suppose the frequencies of the RF, LO and IF signals are represented as fRF, fLO and fIF, respectively. Since fIF, represents a difference between fRF, and fLO, fIF=|fRFxe2x88x92fLO|.
On the other hand, when the frequency converter functions as an upconverter, the IF signal input through the third terminal 7 is converted into the RF signal with a frequency represented as the sum of the frequencies fIF and fLO of the IF and LO signals; |fIF+fLO|=fRF. Then, the RF signal is output through the second terminal 5.
The prior art frequency converter, however, has various shortcomings. Firstly, the frequency conversion performed by the converter is affected by the nonlinear channel resistor Rd▪ to generate second and third harmonics with twice and thrice the frequencies of the fundamental frequency fLO of the LO signal, thus interfering with the frequency conversion by the FET 1.
Accordingly, when the frequencies of the LO, RF and IF signals are 2.2 GHz, 2.0 GHz and 200 MHz, respectively, the conventional frequency converter results in a conversion loss as high as about 7 dB.
Secondly, an LO signal amplifier including another FET usually precedes the first terminal 3 in a telecommunications system and those second and third harmonics are also generated during amplification by the LO signal amplifier. And those harmonics are input to the FET 1, too.
That is to say, the FET 1 is further affected by the additional harmonica produced by the FET on the previous stage. Accordingly, when the frequencies of the LO, RF and IF signals are 2.2 GHz, 2.0 GHz and 200 MHz, respectively, the conversion version loss involved with the conventional frequency conversion is as high as about 8 dB.
It is therefore an object of the present invention to reduce a conversion loss caused by a frequency converter.
A first inventive frequency converter includes: a first terminal through Which a local oscillator signal is input; a second terminal through which an input signal with a frequency to be converted is input; a third terminal through which an output signal with a different frequency resulting from the conversion is output; and a field effect transistor with gate, source and drain terminals for converting the frequency of the input signal and outputting the signal with the different frequency as the output signal. The gate terminal is connected to the first terminal, while the drain terminal is connected to the second and third terminals. The frequency converter further includes a trap circuit, which is connected to the source terminal of the field effect transistor and resonates at a frequency of a harmonic of the local oscillator signal, thereby substantially eliminating the harmonic.
In the first frequency converter, the trap circuit resonates at a frequency of a harmonic of the local oscillator signal, thereby substantially eliminating the harmonic. That is to say, the frequency conversion by the field effect transistor is much less interfered with by the harmonic, thus attaining reduced conversion loss and improved conversion efficiency. Accordingly, supposing the first inventive frequency converter results in a conversion loss at the same level as the prior art converter, the inventive converter can greatly reduce the power level of the LO signal. As a result, this converter can greatly contribute to reduction in power dissipated by a wireless communications system.
In one embodiment of the present invention, the trap circuit preferably includes: an LC serial circuit consisting of an inductor and a capacitor that are connected in series to each other; and a resistor connected in parallel to the LC serial circuit. In the trap circuit, f=1/(2xcfx80xc3x97(LC)xc2xd) is preferably met, where f is the frequency of the harmonic of the local oscillator signal, L is an inductance of the inductor and C is a capacitance of the capacitor. And one terminal of the trap circuit is preferably connected to the source terminal of the field effect transistor, while the other terminal of the trap circuit is preferably grounded.
In such an embodiment, the trap circuit resonates at a frequency of a second harmonic of the local oscillator signal. Accordingly, the frequency conversion performed by the field effect transistor is much less interfered with by the second harmonic.
A second inventive frequency converter includes: a first terminal through which a local oscillator signal is input; a second terminal through which an input signal with a frequency to be converted is input; a third terminal through which an output signal with a different frequency resulting from the conversion is output; and a field effect transistor with gate, source and drain terminals for converting the frequency of the input signal and outputting the signal with the different frequency as the output signal. The gate terminal is connected to the first terminal, while the drain terminal is connected to the second and third terminals. The converter further includes a trap circuit, which is connected to the gate terminal of the field effect transistor and resonates at a frequency of a harmonic of the local oscillator signal, thereby substantially eliminating the harmonic.
In the second frequency converter, the trap circuit resonates at a frequency of a harmonic of the local oscillator signal, thereby substantially eliminating the harmonic. That is to say, the frequency conversion performed by the field effect transistor is much less interfered with by the harmonic. In addition, the amplification performed by another field effect transistor, which is provided at a stage preceding the first terminal for amplifying the LO signal, is also much less interfered with by the harmonic, thus attaining far lower conversion loss and much higher conversion efficiency. Accordingly, supposing the second inventive frequency converter results in a conversion loss at the same level as the prior art converter, the inventive converter can greatly reduce the power level of the LO signal. As a result, this converter significantly contributes to further reduction in power dissipated by a wireless communications system.
In one embodiment of the present invention, the trap circuit preferably includes an LC serial circuit consisting of an inductor and a capacitor that are connected in series to each other. In the trap circuit, f=1/(2xcfx80xc3x97(LC)xc2xd is preferably met, where f is the frequency of the harmonic of the local oscillator signal, L is an inductance of the inductor and C is a capacitance of the capacitor. And one terminal of the trap circuit is preferably connected to the gate terminal of the field effect transistor, while the other terminal of the trap circuit is preferably grounded.
In such an embodiment, the trap circuit resonates at a frequency of a second harmonic of the local oscillator signal. Thus, both the frequency conversion by the field effect transistor and the amplification by another field effect transistor are much less interfered with by the second harmonic.
In an alternative embodiment, the trap circuit may includes an LC parallel circuit consisting of an inductor and a capacitor that are connected in parallel to each other. In the trap circuit, f=1/(2xcfx80xc3x97(LC)xc2xd) is also preferably met, where f is the frequency of the harmonic of the local oscillator signal, L is an inductance of the inductor and C is a capacitance of the capacitor. And one terminal of the trap circuit is preferably connected to the gate terminal of the field effect transistor, while the other terminal of the trap circuit is preferably connected to the first terminal.
In such an embodiment, the trap circuit resonates at a frequency of a second harmonic of the local oscillator signal. Thus, both the frequency conversion by the field effect transistor and the amplification by another field effect transistor are much less interfered with by the second harmonic.